Recovery of petroleum from a subterranean reservor



lab ld f3 FIPYElFf Nov. 22, 1966 J. P. HELLER RECOVERY OF PETROLEUM FROMA SUBTERRANEAN RESERVOIR Filed Sept. 16. 1963 5 Sheets-Sheet l PRIOR ART.JOHN P HELLER INVENTOR JNM 2". 99M

ATTORNEY I V- ,1966 J. P. HELLEE 3,286,768

- Filed Sept. 16. 1963 RECOVER! OF PETROLEUM FROM A SUBTERRANEANRESERVOIR 3 Sheets-Sheet 2 a Blusz JOHN P/HELLER INVENTOR.

BYJMJ -A,

ATTORNEY Nov. 22-, 1966 J. P. HELLER 3,236,763

- RECOVERY OF PETROLEUM FROM A SUBTERRANEAN RESERVOIR Filed Sept. 1s.1963 a Sheets-Sheet 5 JOHN RHELLER INVENTOR.

Of Mai ATTORNEY the number of injection and production wells.

'tionally, the injection and productionwells are arranged in regular,uniform patternsi While high areal sweep RECOVERY OF PETROLEUM FROM ASUBTERRANEAN RESERVOIR John P. Heller, Dallas, Tex., assignor to Mobiloll Corporation, a corporation of New York Filed Sept. 16, 1963-,Sen-No. 309,123 14 Claims. (Cl. 166-9) -terranean formations, or'reservoirs, as a result of gas pressure or natural water driveforcingthe petroleum from the petroleum-bearing formation to a producing welland then to the surface of the earth. As recovery of petroleum from theformation continues, the reservoir energy gradually decreases. A majorportion of the peltroleum still remains in the formation "and at anytime after the reservoir energy begins to decline, secondary recoverymethods may be initiated to increase the recovery of this remainingpetroleum. Among these methods are those which involve flooding theformation with a liquid to displace the petroleum from the formation anddrive it to at producing 'well. Flooding is also often employedinitially for the production of petroleum where the formation does notcontainsuflicient reservoir energy to produce the petroleum. Y

In recovery methods. employing a displacing liquid, the

'shape of the displacement front, i.e., the advancing front of thedisplacing liquid'in the formation surrounding each injection well, isdetermined by various factors. Included among these are inhomogeneities-of the formation, the number and relative position of the injection andproduction wells, and the fluid properties of the displaced liquid andthe displacing liquid. The shape of the area through which thedisplacement front has passed is termed the sweep pattern and the ratioof the area within the sweep pattern to the total area is termed thesweep efiiciency. Practical operations, from the standpoint of economy,require a maximum sweep efficiency commensurate with Convenellicienciesare obtained with these well patterns, and with others which do not haveregularity or uniformity, still higher efiiciencies are desirable.

It is an object of this invention to improve the extent of recovery ofpetroleum from a subterranean formation.

It is another object of this invention to improve the sweep efliciencyin an operation for the recovery of petroleum from a subterraneanformation employing a displacing liquid.

It is another object of this invention to provide a method forcontrolling the shape of the displacement front passed through asubterranean formation containing petroleum.

Further objects of the invention will become apparent from the followingdetailed description,

In accordance with the invention, there is provided a procedure whichinvolves the creation within a formation containing petroleum of aninstability finger of a displacing liquid in a direction from theinjection well to a production well in which the flow would normally bethe slowest.

FIGURE 1 is a plan view of a portion of a petroleum fielddiagrammatically illustrating an arrangement of inectlon wells andproduction wells and the position and ,the shape of the displacementfronts of the displacing llquld in ected into the injection wells inaccordance with conventional procedure.

F United States atone Patented Now-22, 1966 fronts of the. displacingliquid injected into the injection wells in accordance with theprocedure of the invention.

FIGURE 3 is another plan view diagrammatically illustrating anarrangement of an injection well and production wells anddiagrammatically illustrating a means for -creating an instabilityfinger of displacing liquid.

FIGURE 4 is a sectional view of the injection well illustrated in FIGURE3.

FIGURE 5 is a plan viewof another means for creating four symmetricallyoriented instability fingers of a displacing liquid.

Referring now to FIGURE 1, the numeral 10 designates a portion of afield having a formation containing petroleum. The field 10 is providedwith injection wells for the injection into the formation of adisplacing liquid and is provided with production wells for the recoveryof the petroleum displaced from the reservoir by the injected floodingliquid. Injection wells 11, 12, 13, and 14 and production wells 15, 20,21, 22, 23, 24, 25, 30, and 31 are shown. In a flooding operation forrecovery of the petroleum within the formation the displacing liquid isinjected into the injection wells 11, I2, 13, and 14.

In recovery methods employing a displacing liquid, the progress of thedisplacement process may be visualized in terms of the motion ofdisplacement fronts. Surrounding each injection well, followinginjection of some of the displacing liquid, there exists a volume regionof the formation within the pores of which the petroleum saturation isrelatively low in consequence of the replacement of the petroleum by thedisplacing liquid in that region. Further out in the formation, there isa region in which the numerical value of the petroleum saturation isunchanged from that which obtained prior to the start of the injection.Lying between those two regions lies a transition zone in which thepetroleum saturation depends relatively sensitively on position and ontime as the displacement proceeds. The closed surface marking the mediansaturation within the transition zone is the displacement front.

The shape of the displacement front surrounding each injection well, asstated, is determined by various factors. First there are the variousgeometrical factors. These include the variations, from place to placein the formation, of porosity, permeability, native fluid saturation andsurface properties of the rock matrix. These may be called the formationinhomogeneities, and may include random variations of the matrix, frommicroscopic poresized dimensions up to large scale trends in the rockproperties over the gross dimensions of the formation. There may also behighly ordered variations in the rock parameters, such as a stronglayered structure. A further set of geometrical factors involved in thedetermination of the shape of the displacement front are the relativepositions of the injection and production wells and of the shape of thegross formation boundaries.

In addition to these geometrical determining factors, the shape of thedisplacement front may also be under the influence of the fluidproperties. This will occur if there is a difference in mobility ordensity between the displaced liquid and the displacing liquid.Variations in frontal shape may then themselves effect changes in thevelocity of fluid movement. The displacement front shape thus becomesunstable in the event the mobility of the displacing liquid is higherthan that of the displaced liquid. In this case the displacement frontdevelops extensions or fingers. As the displacement process or flood iscontinued, these will break through into the production well, dilutingthe produced liquid with displacing liquid and ldecreasing the lifetimeof the field during which petroleum may be produced economically. Justafter the start of the injection program, the displacement frontssurrounding the injection wells are relatively smooth and reflect theshape of the input boundaries which are injection well faces. This isthe case even in floods wit-h an adverse mobility ratiothat is, in whichthe ratio of the fluid mobility of the displaced phase to that of thedisplacing phase is greater than unity, leading to the aforementionedinstability fingering. As the flood progresses, however, thedisplacement front develops irregularities corresponding to thegeometrical factors mentional above. in the unfavorable mobilitysituation these irregularities are amplified :by the instabilitymechanism and develop into finigers which reduce the sweep efliciency.

Assuming that the formation is more or less homogeneous and assumingfurthermore that the displacement from i, 12,- 13, and 14 and thedisplacement front between the petroleum and the displacement front will'be in t'he'fcrrn 'of a circle such as circles 32, 33, 34, and 35.Further from the injection wells, the streamlines depart from theirinitially radial directions. The details of these deviations will dependupon the position of the production wells with respect to the'injectionwells. More specifically, the position of the production wells withrespect to the injection wells will determine the direction of thepathways of the greatest pressure differential between the injectionwell and each of the production wells. The liquid passed into theinjection wells will flow in these directions at a rate greater thanthat in any other direction in the formation between the injection wellsto the production wells.

Wit-h a homogeneous formation, the pathway of greatest pressuredifferential between injection and production wells will be in thedirection of a straight line between the injection well and each of theproduction wells. As a consequence, the displacing liquid will flow atthe highest rate from the injection well directly toward the productionwells and at the lowest rate in a direction from the injection well to apoint midway on a straight line between adjacent production wells. Withcontinued injection of the displacing liquid, the pattern of thedisplacement front will thus have the form of the patterns 40, 41, 42,and

43, for each of the injection wells 11 to 14, respectively. .Withcontinued injection of the displacing liquid, breakthrough of theflooding liquid will eventually occur at the production wells and atbreakthrough the displacement front will have the form of patterns 44,45, 50, and 51 for each of the wells 11 to 14, respectively.

The shape of the displacement fronts as described above placing liquid.Thus, as shown in FIGURE 1, the areas 52, 53, 54, and 55 have not beenswept. Accordingly, the petroleum contained in the formation withinthese areas is not recovered at breakthrough. To the extent that thispetroleum is not recovered, the efficiency of the flooding operation isdiminished. Frequently, the efiiciency of the flooding operation, fromthe standpoint of the areas swept by the displacing liquid, maybe onlyabout 50 percent.

By the procedure of the invention, the sweep efficiency of thedisplacing liquid is improved. With reference to FIGURE 2, the numeral60 design-ates a portion of a field having a. formation containingpetroleum. The field 60 is provided with injection wells 61, 62, 63, and64. Additionally, the field is provided with production wells 65,

70, 71, 72, 73, 74, 75, 80 and 81. Displacing liquid is passed into eachof the injection wells 61 to 64. Howassumes, as mentioned, homogeneityof the formation and stability of the fronts. In practical operations,the formation may not the homogeneous. Further, as the result of anunfavorable mobility ratio and inhomogeneities in the formation,instability fingers of the displacing liquid can form. The earlyinstability fingers are formed haphazardly in directions dictated byminor variations of rock properties. Their formation can result inpremature breakthrough of the displacing liquid at any one of theproduction wells surrounding an injection well. The instability willalso cause an earlier development of the enhanced flow along thediagonals between injection and production wells. Thus, the shape of thedisplacement fronts as described above will 'be changed. However, it canbe considered that from an overall standpoint the shape of thedisplacement fronts will be more or less as described.

It will be seen from FIGURE 1 that, at breakthrough of the displacingliquid at the output wells, an appreciable ever, in the process of theinvention, appropriately onenbed instability fingers of the displacingliquid are created within the formation. These instability fingers arecreated by introducing the displacing liquid from the injection wellinto the formation in those directions in which the flow of thedisplacing liquid from the injection well to the production well isnormally the slowest. In the S-spot pattern of injection and productionwells set forth in FIG- URE 2, the directions of lowest rate of flow ofthe displacing liquid from the injection well to each of the fourproduction wells surrounding each injection well, assuming homogeneityof the formation, are to those points midway between adjacent productionwells. Stated otherwise, the directions of lowest crate of flow of thedisplacing liquid are at the angle of 45 from the line joining theinjection well with the respective production well.

In the embodiment set forth in FIGURE 2, four instability fingers arecreated within the formation for each of the injection wells 61 to 64.These fingers are created at an angle of with respect to each other andeach is created in a direction on a line 45 from the line joining theinjection well and each of the four production wells associated with theinjection well. Thus, for each of wells 61 to 64, shortly after thebeginning of injection of the displacing liquid, the shape of thedisplacement front will have the form of patterns 32, 83, 84, and 85.Each of these patterns, with particular reference to pattern 82-as anexample, will contain four fingers 90, 91, 92, and 93. Thedisplacingliquid is continued to be injected into the formation in the directionof the lowest rate of flow and, with the continued injection of thedisplacing liquid, the shape of the fingers will become less clearlydefined. However, the effect of the creation of the fingers along thedirection of the lines of slowest flow of the displacing liquid in theformation will be maintained. The influence of the position of theproduction wells with respect to the injection wells on the direction ofthe flow of the displacing liquid from the injection well will becomeprogressively greater as further displacing liquid is passed into theformation.

On the other hand, the initial formation of the instability fingers andcontinued injection of the displacing liquid would not otherwise havebeen swept. Accordingly, with placing liquid to sweep portions of theformation that continued injection of the displacing liquid, thedisplacement front will eventually have the shape of patterns )5, 100,101, and 102. It will be seen from these patterns that the displacingliquid retains a tendency to flow in the direction of the createdinstability fingers. This tendency is a consequence of the adversemobility ratio. The higher this ratio is, the greater will the tendencybe. Eventually, however, the trend of movement of the displacing liquidin the direction of the created inst-ability fingers will be reversed sothat the direction of greatest rate of fiow will be along the linesdirectly between the injection and the production wells. Thus, at breakthrough, the pattern of the displacement front will have the shape ofthe patterns 103,104, 105, and 110. It

, and 132.

will be seen that, at breakthrough, the pattern of the swept area of theformation is greater than the pattern of the swept area indicated. inFIGURE 1. Thus, the unswept portions 111, 112, 113, 115, 120, 121, and122 of the formation are considerably smaller than the unswept portions52, 53, 54, and 55 in FIGURE 1. Thus, by the process of the invention,the area of the formation unswcpt by the displacing liquid may be onlyto 20 percent of the total area of the formation. Where the fingers fromadjacent injection wells meet at or before breakthrough of thedisplacing liquid at the production wells, nearly complete sweepoutwould be expected.

The process of the invention has been described above in connection withinjection into a single injection well and production from fourproduction wells equidistantly spaced from .each other and from theinjection well. However, the process of the invention may also beapplied where nhere are a different number of injection wells and adifferent number of production wells. Further, it may be applied wherethe wells, whether injection or production wells, are not equidistantlyspaced from each other. Ordinarily, assuming homogeneity of theformation, the direction of the highest rate of flow of the displacingliquid from an injection well to a production well is that of a straightline between the injection well and the production well. Thus,ordinarily, still assuming homogeneity of the formation, the directionof the lowest rate of flow of the displacing liquid is that of astraight line rbisecting the angle from the injection well swept by astraight line from a production well toan intercepted production well..For example, where there are one injection well and one productionwell, the angle from the injection well swept by a straight line fromthe production well to the next intercepted production well, i.e., thesame production well, is 360". A straight line bisecting this angle willbe oriented in a direction of 180 and therefore the direction of thelowest rate'of flow will be opposite to that of the highest rate offlow. Similarly, for example, in the emdirection of lowest rate of fiowis that of the line bisecting this angle.

It may occasionally be that the format-ion is not homogeneous. Where itis known that the formation is not homogeneous, the direction of thelowest rate of flow of displacing liquid will often also be known. Inany case, the direction of the lowest rate of flow of displacing liquidmay be determined. In accordance with another feature of the invention,prior to injection of the displacing liquid into the formation throughthe injection well, the direction of slowest flow of the displacingl-iquid'through the formation between the injection well and theproduction well is determined. Knowing the direction of the lowest rateof flow of the displacing liquid, the displacing liquid is injected intothe formation in this direction.

Various methods for creating an instability finger of the displacingliquid may be employed. In a preferred embodiment of the invention, theinstability finger can be created by injecting the displacing liquidthrough the injection well and into the formation through a direc-=tionally oriented void created within the formation from the wall ofthe well. In a particularly preferred embodirhent of the invention, theinjection well is perforated employing conventional gun perforatingmeans to create the directionally oriented void. FIGURES 3 and 4, re;spectively, illustrate this latter preferred embodiment of theinvention.

Referring to FIGURES 3 and 4, injection well 123 penetrates a portion ofthe formation 124. The formation is provided with production wells 125,130, 131, Imaginary line 133 connects wells 125 and 130, imaginary line134 connects wells 130 and 131. imaginary line 135 connects wells 131and 132, and

- imaginary line 140 connects wells 132 and 125. For

purposes of illustration, well 123 is shown as being en larged withrespect to the production wells.

Well 123 is provided with casing 141 extending to the bottom of the wellwherein it is held in place by cement liner 142. The well is'gunmerforated in four directions to provide four"- voids 143, 144, 145and 150. Void 144 is oriented as to point in the direction of themidpoint of line 133. Similarly, voids 145, 150, and 143 are oriented asto point in the direction of the midpoints of lines 134, 135, and 140,respectively. The voids penetrate the easing 141 and the cement liner142 and enter into the forma' tion 124;. Upon injection of displacingliquid into well 123, the liquid passes from the well through the voids143, 144, 145, and into the formation and creates an instability fingeroriented in the direction of the-midpoint of the lines joining theproduction wells.

Referring now to FIGURE 5, there is disclosed another means whereby aninstability finger may be created in the formation in the desireddirection of flow of the displacing liquid. Referring now to FIGURE 5,injection well 151 penetrates formation 152. Disposedlongitudinallywithin the well is distribution tool 153. This tool 153comprises an octagonally shaped casing 154 provided on its outer surfaceby flow dividers 155, 160, 161, 162, 163, 164, 165, and 170. Each ofthese flow dividers is adapted to contact the wall of the well 151.Extending longitudinally through the distribution tool is a pair ofpipes 171 and 172. Extending from pipe 171 are flow conduits 173, 174,175, and 180. Extending from pipe 172 are flow conduits 181, 182, 183.and 184. In operation, the flow dividers 155, to 165, and separate thewall of the well 153 into eight segments. The tool 153 is oriented inthe well such that four alternate segments are pointed into thedirection in the formation in which it is desired to create theinstability fingers. Depending upon the direction into which theinstability fingers are to be created, the

displacing liquid is passed through either pipe 171 or 1721.

The displacing liquid flows through the flow conduits associated withthe pipe and enters alternate segments formed by the flow dividers; Theliquid then enters the formation from the segments" to create the fourinstability fingers.

In the practice of the invention, thedisplacing liquid has a mobilityhigher than the mobility of the petroleum in the formation. relativepermeability of the formation to the liquid to the viscosity of theliquid.- With the displacing liquid having the higher mobility, themobility ratio of the displacing liquid to the displaced liquid is suchas to impart instability to the displacement front and maintain the ofthe created instability fingers.

The procedure of the invention may be employed in connection withflooding operations in which the displacing liquid is miscible with thepetroleum in the formation. For example, the procedure of the inventionmay be cmploycd .in connection with the injection of liquefied petroleumgas (LPG) or other light hydrocarbon which is miscible with thepetroleum in the formation. In these operations, following injection ofthe displacing liquid miscible with the petroleum in the formation, adriving fluid is employed to drive the miscible displacing liquidthrough the formation from the injection well into the direction of theproduction wells. The driving fluid, which may be a gas, can be misciblewith the displacing liquid.

The procedure of the invention may also be employed where the displacingliquid is immiscible with the petroleum in the formation. Thus, forexample, water may be injected into the formation to drive the petroleumthrough the formation from the injection well to the production well.

The displacing liquid injected into the formation from By mobility ismeant the ratio of the the injection well, whether miscible orimmiscible with the petroleum in the formation, is also capable, aspreviously mentioned, of forming haphazard instability fingers.

,T'hese haphazard instability fingers are of higher wave number. Whilethe formation of these more closely spaced haphazard instability fingersin the present process fingers can be prevented by avoiding largeviscosity gradicnts between the petroleum in the formation and thedisplacing liquid. These large viscosity gradients may be avoided byemploying as the displacing liquid a mixture of petroleum from theformation and a conventional flooding liquid such as liquefied petroleumgas. As this displacing liquid is injected into the formation, theproportion oil with respect to the amount of liquefied petroleum gas orother conventional displacing liquid is continually reduced so that atthe completion of injection of a graded zone" of displacing liquid, thedisplacing liquid is substantially or completely all liquefied petroleumgas. More specifically, the flooding operation may be carried outemploying at the beginning of operation a mixture of 9 parts ofpetroleum from the formation and 1 part of liquefied petroleum gas.Subsequently, the displacing liquid will L consist of 8 parts ofpetroleum and 2 parts of liquefied petroleum gas. Thereafter, theviscosity of the displacing liquid may. be reduced further by reductionin the proportion of petroleum by 1 volume and increase in theproportion of liquefied petroleum gas by 1 volume until the displacingliquid is all liquefied petroleum gas. Where a fluid immiscible with thepetroleum in the formation is employed, various types of thickeningagents may be added to the liquid to effect a gradation in viscosity andthus avoid large viscosity differences within the formation between theliquid therein and the displacing liquid.

Having thus described my invention, it will be understood that suchdescription has been given by way of illustration and example and not byway of limitation, reference for the latter purpose being had to theappended claims.

I claim:

1. In a process for the production of petroleum from a formationpenetrated by an injection well and a production means including atleast one production well by injection of a displacing liquid into saidformation through said injection welland displacement of said petroleumby said displacing liquid in the direction of said production well, theimprovement comprising injecting said displacing liquid into saidformation from said injection well only in a direction in which flow onthe displacing liquid would normally be the slowest from said injectionwell to said production well.

2. The procedure of claim 1 wherein said displacing liquid has amobility higher than the mobility of said petroleum in said formation.

3. The procedure of claim 1 wherein said displacing liquid is immisciblewith said petroleum in said formation.

4. The procedure of claim 1 wherein said displacing liquid is misciblewith said petroleum within said formation.

5. The procedure of claim 1 wherein said formation is penetrated by aninjection well surrounded by four production wells.

6. The procedure of claim 1 wherein said injection well is subjected togun-perforation prior to injection of said displacing liquid to create avoid into said formation from said well oriented in a direction in whichflow of the displacing liquid would normally be slowest from saidinjection well to said production well and injecting said displacingliquid into said formation through said void.

7. The procedure of claim 1 wherein said displacing liquid has a gradedviscosity with respect to said petrolcum in said formation whereby sharpviscosity gradients and the production of extraneous, closely-spacedinstability fingers are avoided.

8. In a process for the production of petroleum from a-formationpenetrated by an injection well and a. production means including atleast one production well by injection of a displacing liquid into saidformation through said injection well and displacement of said petroleumby said displacing liquid in the direction of said production means, theimprovement comprising injecting said displacing liquid into saidformation from said injection well only in the direction of a linebisecting the angle from said injection well swept by a line betweensaid injection well and a production well to the first interceptedproduction well.

9. In a process for the production of petroleum from a formationpenetrated by an injection well and a plurality of production wellsequidistantly spaced from each other and from said injection well byinjection of a displacing liquid into said formation through saidinjection well and displacement of said petroleum by said displacingliquid in the direction of said production wells, the improvementcomprising injecting said displacing liquid into said formation fromsaid injection well only in a direction to a point located midway on aline between adjacent production wells.

10. In a process for the production of petroleum from a formationpenetrated by an injection well and four production Wells equidistantlyspaced from each other and from said injection well by injection of adisplacing liquid into said formation through said injection well anddisplacement of said petroleum by said displacing liquid in thedirection of said production wells, the improvement comprising injectingsaid displacing liquid into said formation from said injection well onlyin a direction to a midpoint of each of the lines joining adjacentproduction wells.

11. In a process for the production of petroleum from a formationpenetrated by an injection well and a plurality of production wellsequidistantly spaced from each other and from said injection welllay-injection of a displacing liquid into saidformation through saidinjection well and displacement of said petroleum by said displacingliquid in the direction of said production wells, the improvementcomprising the steps of creating a plurality of voids into saidformation from said injection well in number equal to the number of saidproduction wells,

12. In a process for the production of petroleum froma formationpenetrated by an injection well and four production wells equidistantlyspaced from each other and from said injection well by injection of adisplacing liquid into said formation through said injection well anddisplacement of said petroleum by said displacing liquid in thedirection of said production wells, the improvement comprising the stepsof creating four voids into said formation from "said injection well,each of said voids being oriented in a direction only to a point midwayof a line joining a different pair of adjacent production wells, andinjecting said displacing liquid into said formation only through saidvoids.

13. In a process for the production of petroleum from a formationpenetrated by an injection well and by a production means including atleast one production well by injection of a displacing liquid into saidformation through said injection well and displacement of said petroleumby said displacement liquid in the direction of said production well,the improvement comprising the steps of determining the direction of thelowest rate of liquid flow in said formation between said injection welland said production well and injecting said displacing liquid into saidformation from said injection well only in said-direction of said lowestrate of liquid flow.

14. In a process for the production of petroleum from a formationpenetrated by an injection well and by a production means including atleast one production well by injection of a displacing liquid into saidformation through said injection well and displacement of said petroleumby said displacing liquid in the direction of said production well, theimprovement comprising determin- 1 ing the direction of the lowest rateof liquid flow in said formation between said injection well and saidproduction well, creating a void from said injection well into saidformation oriented only in said direction of said lowest rate of liquidflow, and injecting displacing liquid into said formation only throughsaid void.

References Cited by the Examiner v UNITED STATES PATENTS 2,107,0072/1938 Lang 166-10 CHARLES E. OCONNELL, Primary Examiner.

T. A. ZALENSKI, S. I. NOVOSAD,

- 7 Assistant Examiners.

1. IN A PROCESS FOR THE PRODUCING OF PETROLEUM FROM A FORMATIONPENETRATED BY AN INJECTION WELL AND A PRODUCTION MEANS INCLUDING ATLEAST ONE PRODUCTION WELL BY INJECTION OF A DISPLACING LIQUID INTO SAIDFORMATION THROUGH SAID INJCTION WELL AND DISPLACEMENT OF SAID PETROLEUMBY SAID DISPLACING LIQUID IN THE DIRECTION OF SAID PRODUCTION WELL, THEIMPROVEMENT COMPRISING INJECTING SAID DISPLACING LIQUID INTO SAIDFORMATION FROM SAID INJECTION WELL ONLY IN A DIRECTION IN WHICH FLOW ONTHE DISPLACING LIQUID WOULD NORMALLY BE THE SLOWEST FROM SAID INJECTIONWELL TO SAID PRODUCTION WELL.